A vehicle driven by a motor such as an electrical vehicle may be equipped with an in-wheel motor system in which a motor is incorporated within a wheel. In such vehicle, since an output of the motor decreases along with the increase of the temperature of the motor while the vehicle is being driven, a cooling apparatus for cooling the motor may be provided.
For example, JP2007-191027A (Reference 1) and JP2007-191035A (Reference 2) each disclose an in-wheel motor system including a geared motor in which an electrical motor is arranged in the rear of a reduction gear mechanism. According to the in-wheel motor system disclosed, the geared motor is supported at a knuckle in an elastic manner by means of a dynamic vibration absorber. An output shaft of the electrical motor includes a hollow portion where an oil introduction passage is formed. A motor case is accommodated within a cylindrical-shaped housing to thereby form a stator cooling passage. The stator cooling passage and an inside of the reduction gear mechanism are connected to each other via an oil passage. A gear oil sent through a pipe and the oil introduction passage from an oil supply apparatus provided at a vehicle body is circulated inside the reduction gear mechanism and the stator cooling passage. As a result, a mass of the gear oil is added to a mass of a dynamic damper. The motor is effectively cooled without increasing a size of the motor accordingly.
In addition, JP2007-215311A (Reference 3) discloses a cooling apparatus of an in-wheel motor. According to the cooling apparatus disclosed, a stator and a cooling passage through which a refrigerant flows are thermally connected to each other. A capacitor for changing the phase of the refrigerant that is in the gas phase at the cooling passage to the liquid phase is connected to a downstream side of the cooling passage and is also arranged at a vertically downward side of the cooling passage upon assembly onto a vehicle. Then, a check valve is disposed between the cooling passage and a refrigerant return passage for returning the refrigerant that is in the liquid phase at the capacitor to an upstream side of the cooling passage. Another check valve is disposed between the capacitor and the refrigerant return passage. As a result, the refrigerant is self-circulated to thereby achieve a downsizing of the cooling apparatus and enhance the cooling performance of the refrigerant.
According to the in-wheel motor system disclosed in each of References 1 and 2, the oil supply apparatus serving as a cooling apparatus is not configured to follow a positional variation of the motor upon operation of a tire, shock absorption, or the like. The pipe connecting the oil supply apparatus and the motor is required to absorb the positional variation of the motor. As a result, an area for allowing or permitting the positional variation of the motor is limited. In addition, the in-wheel motor system disclosed in each of References 1 and 2 does not include means for cooling the oil of which temperature has increased, which may prevent effective cooling of the motor.
According to the cooling apparatus of the in-wheel motor disclosed in Reference 3, the capacitor for air-cooling the refrigerant is arranged at an inner peripheral side of the tire, which may prevent sufficient air supply and air-cooling of the refrigerant. In addition, since the capacitor is arranged at an outer peripheral side (i.e., vertically downward side) of a housing of the motor, the size of the motor should be reduced, which may prevent a use of the motor having a high output level. Further, a sufficient refrigerant volume may not be obtained because of the size of the motor. In such case, heat transfer efficiency from a motor coil to the housing deteriorates, thereby increasing the temperature of the motor. Furthermore, since the capacitor is exposed to the outside of the tire, the capacitor may be damaged by flying gravel while the vehicle is being driven.
A need thus exists for an in-wheel motor system which is not susceptible to the drawback mentioned above.